Background: Up to now there is no study evaluating correlation between serum magnesium and morbidity or mortality in patients admitted in intensive care unit. The aim of this study is to determine the prevalence of hypomagnesemia in critically ill patients and to evaluate its association with organ dysfunction, hospitalization period and mortality. Methods: We conducted a retrospective trial including 100 patients aged 16 years and above admitted to the medico-surgical intensive care unit over 2 years. Firstly, total serum magnesium was determined and then its correlation with ventilator need, duration of mechanical ventilation, hospitalization period, and demographic characteristics were evaluated. Results: At the time of admission, 51% and 49% of the patients showed hypomagnesia and normomagnesia, respectively. Significant difference was observed in mortality rate, hospitalization period, or hospitalization in intensive care unit between the two groups (P<0.05, each). Hypocalcemia, hypokalemia, and hyponatremia were frequently observed in hypomagnesemic patients. The patients with hypomagnesemia in intensive care unit demonstrated higher Acute Physiology And Chronic Health Evaluation II (APACHE-2( and Sequential Organ Failure Assessment (SOFA) scores at time of admission (P<0.01), a higher maximum SOFA score during hospitalization in intensive care unit, higher requirement to ventilator, and longer duration to mechanical ventilation than the other patients. The ROC curve of SOFA score in the hypomanesemia generated significant results compared to APACHE-2. An increase of 5 units in the APACHE-2 or SOFA measured during admission increased relative probability of hypomagnesemia by 0.12 and 0.16, respectively. Conclusion: Hypomagnesemia during hospitalization in intensive care unit was associated with a poor prognosis. Monitoring of serum magnesium may have prognostic and therapeutic implications.